The present invention relates generally to a system and method for opening at least one valve in an internal combustion engine. More specifically the invention relates to a system and method, used both during positive power and engine braking engine operating conditions, for controlling the amount of xe2x80x9clost motionxe2x80x9d between the at least one valve and an assembly for opening the at least one valve.
Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. The exhaust valves may be opened to allow combustion gas to escape from the cylinder.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator with increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a xe2x80x9clost motionxe2x80x9d device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly. In a lost motion system, a cam lobe may provide the xe2x80x9cmaximumxe2x80x9d (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
In the lost motion system of U.S. Pat. No. 5,680,841, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Previous lost motion systems have typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system. Lost motion systems of the prior art have accordingly not been variable such that they may assume more than one length during a single cam lobe motion, or even during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
The lost motion system and method of the present invention may be particularly useful in engines requiring valve actuation for both positive power and for compression release retarding and exhaust gas recirculation valve events. Typically, compression release and exhaust gas recirculation events involve much less valve lift than do positive power related valve events. Compression release and exhaust gas recirculation events may however require very high pressures and temperatures to occur in the engine. Accordingly, if left uncontrolled (which may occur with the failure of a lost motion system), compression release and exhaust gas recirculation could result in pressure or temperature damage to an engine at higher operating speeds. Therefore, it may be beneficial to have a lost motion system which is capable of providing control over positive power, compression release, and exhaust gas recirculation events, and which will provide only positive power or some low level of compression release and exhaust gas recirculation valve events, should the lost motion system fail.
An example of a lost motion system and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such system is not variable within an engine cycle.
The development of lost motion systems has also lead to the integration of such systems into existing engine components, as opposed to adding such systems aftermarket. One particular form of system integration that appears desirable is the integration of the lost motion system into an engine rocker arm, such as is shown in Hu, U.S. Pat. No. 5,680,841. By integrating the lost motion system into the engine rocker arm, savings in weight, size, and cost may be available.
All of the foregoing developments, such as high speed lost motion actuation, and rocker arm integration, have necessitated independently and collectively, smaller, faster, more robust, more controllable, and more compliant lost motion components. One such component that requires improvement to meet the needs of these new and advanced lost motion systems is the system accumulator.
Lost motion systems may require the use of an accumulator to absorb hydraulic fluid that is quickly shuttled into and out of the system, as well as to handle the rapid pressure changes (i.e. from high pressure to low pressure and visa-versa) that occur in the system as a result of high speed actuation. The very nature of accumulators dictates that they be sufficiently robust to withstand high and rapidly changing pressures. Compliance issues also require that the accumulators be located as closely as possible to the lost motion element with which they are in hydraulic communication. Compliance issues also mandate that the lost motion system, and to some degree, the accumulator, be adapted to bleed air from the working fluid thereby reducing the compressibility of the fluid.
Locating an accumulator near a lost motion element, particularly one integrated into an engine rocker arm, constrains the size and weight of the accumulator, which in turn affects the designers ability to make the accumulator robust. There is a natural inverse relationship between the robustness of an accumulator and its size and weight. The smaller and lighter the accumulator, the less robust it tends to be. Thus, the combination of loading and space requirements of accumulator pistons associated with integrated engine brakes provides a challenge to engine brake designers. In view of the foregoing, there is a need for an accumulator that is reduced in size, cost effective, sufficiently robust, capable of bleeding air, and controllable.
It has been determined that control over the amount of hydraulic fluid that the accumulator is designed to accumulate may be particularly important to the operation of the lost motion system. Without precise accumulator control, an engine valve may experience over-travel or under-travel. Moreover, imprecise accumulator control may have a negative impact on control and consistency of engine valve seating timing and velocities.
Engine valve over-travel during main events may result in valve to piston contact or the need for valve pockets in the piston. Neither valve to piston contact, nor valve pockets are desirable. Under-travel may lead to ineffective auxiliary valve events, such as compression-release events, or ineffective overlap between main intake and exhaust events. In order to reduce the likelihood of valve over-travel or under-travel, and to provide desirable valve seating timing and velocities, Applicant has developed an accumulator that absorbs a predetermined fixed volume of hydraulic fluid upon each actuation cycle of the engine brake. This accumulator provides the ability to lose the precise amount of motion provided by an engine brake lobe, or another auxiliary lobe on the exhaust cam. The loss of this precise amount of motion permits the engine valve to seat consistently, and the engine piston to be provided without pockets, while avoiding the likelihood of valve to piston contact.
Accumulator design must also take into account the undesired heating of the hydraulic fluid used in the lost motion system. Typically, engine oil is used as the working hydraulic fluid. Such engine oil enters the system already somewhat heated due to its use in the operation of the engine. The oil in the lost motion system is further heated as a result of flowing rapidly through the passages that make up the system. It would therefore be beneficial to provide accumulators with some means of cycling hydraulic fluid through the lost motion system so that there is a constant influx of fresh cool fluid into the system.
In order to provide an accumulator with all of the foregoing beneficial characteristics, Applicant has developed an accumulator that may be integrated into a lost motion piston, such as a slave piston. Such an integrated accumulator saves space and cost due to the use of the slave piston bore as the bore for the accumulator. The integrated accumulator is also capable of being quite robust because it may be manufactured of the same strength steel used for the slave piston.
Applicant has also developed an accumulator capable of providing a precise amount of lost motion clipping of a main engine valve event. Such precise clipping is attained through use of a fixed volume or fixed displacement accumulator. Clipping without a fixed volume may either result in too much, or too little engine valve travel being removed. The later may result in valve-to-piston contact, and the former may cause the valve to be seated at a higher velocity than desired. At a minimum, this may lead to increased engine valve seat wear, and possibly to some form of engine valve failure.
In accordance with embodiments of the present invention, it is contemplated that the accumulator system may be located in a master piston, a slave piston, or separate piston. It is further contemplated that in accordance with the present invention the accumulator system may be located within a rocker arm assembly of an engine rocker brake.
It is therefore an object of the present invention to provide a lost motion system accumulator with improved robustness for its size.
It is another object of the present invention to provide a lost motion system accumulator that reduces accumulator bore wear.
It is another object of the present invention to reduce the package size of a lost motion system accumulator.
It is a further object of the present invention to provide a more cost effective method for packaging a lost motion system accumulator.
It is still another object of the present invention to reduce some of the variances of bleed rate for a lost motion system accumulator due to pressure differentials.
It is yet another object of the present invention to improve braking performance by improving compliance of a lost motion system accumulator.
It is still yet another object of the present invention to provide a lost motion system accumulator capable of venting and/or absorbing a fixed volume of hydraulic fluid to eliminate valve-to-piston clearance issues.
It is still another object of the present invention to provide a lost motion system accumulator with desirable air bleeding and hydraulic fluid circulation capabilities.
It is still another object of the present invention to provide a lost motion system accumulator that will reduce engine valve spring stresses as a result of fixed volume accumulator.
It is still another object of the present invention to provide a lost motion system accumulator that provides lower engine valve seating velocities.
It is still another object of the present invention to provide a lost motion system accumulator that provides more consistent valve seating timing and velocities.
Additional objects and advantages of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.
In response to the foregoing challenge, Applicant has developed an innovative, economical method or system for providing a lost motion accumulator that uses a captive (fixed) volume that can be selectively hydraulically or pneumatically locked, or vented in order to maintain or increase the total volume of the lost motion system.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated herein by reference, and which constitute a part of this specification, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention.